The technical field of this invention is solid state integrated circuit fabrication and, more particularly, methods for fabricating voltage programmable link structures.
Programmable conductive paths, particularly "links" between two or more distinct metallization layers, are increasingly employed in solid-state integrated circuit fabrication to produce a wide variety of programmable circuits including, for example, field programmable gate arrays ("FPGAs"), programmable read only memories ("PROMs"), and other programmable electronic devices.
Most typically, such devices are "programmed" by the application of an electrical voltage to trigger an "antifuse" link structure disposed between two metallization layers and thereby establish an electrical connection across a region of the device which had previously been an insulator.
While this approach should in theory permit an almost limitless variety of custom circuits, certain factors make programmable devices difficult to implement. To be useful, link structures must remain insulating at the normal operating voltage for solid state devices (e.g., nominally five volts), but must reliably "break down," or respond, to a programming voltage which is higher than the normal operating voltage, but not so high as to damage other structures on the circuit (e.g., not more than about fifteen volts).
If a link structure breaks down at a voltage below the programming voltage (or breakdown voltage) an unintended altered circuit will result, thereby disturbing the normal operation of the existing circuit. On the other hand, if the programmable link is over-resistant to the programming voltage, either the conductive path will not be formed when desired, or greater voltages must be applied with the attendant risk of damage to nearby structures on the wafer.
One problem with the fabrication of reliable, voltage programmable, metal to metal, link structures is that aluminum-based metallization (aluminum is almost universally used throughout the integrated circuit industry) has a tendency to form rather marked surface irregularities in the form of "hillocks" and the like, during sintering and other device processing steps. The hillocks are typically on the order of one micrometer in diameter and height. These hillocks can pierce or otherwise damage overlying insulator layers unless such insulator layers are rather thick (e.g., greater than about 500 nanometers). For conventional, multi-level metallization, hillocks are less troublesome because the intermetallic insulators normally used are thick, but fabrication of a link structure with a low programming voltage requires the use of a thin insulator.
The thin insulator layer required for link structures makes link structures particularly sensitive to the occurrence of hillocks. If the insulator layer is too thin, hillocks may damage the insulator thereby lowering the breakdown voltage below normal operating voltages. However, if a thick insulator layer is employed to avoid inadvertent breakdowns, the resulting programming voltage may be so high as to damage other structures on the wafer. Since hillocks do not occur according to a uniform distribution it is extremely difficult to manufacture a reliable link structure with a low programming voltage using aluminum-based metallization.
One way to avoid the problem of hillocks is to use non-metallic conductors, such as polysilicon or doped silicon, instead of aluminum. This approach allows the fabrication of reliable voltage programmable links, but due to the higher resistance of these conductors, such links tend to have an after programming resistance of many hundreds of Ohms, in contrast to links formed with aluminum conductors which can have a resistance of as low as 1 Ohm after programming.
Moreover, silicon and polysilicon are not suitable in many applications to make long conductors because of their high resistivity. Thus, if silicon or polysilicon-based conductors are used in link structures, connections must be made to metal layers. These connections or "contacts" must be relatively large, which limits the density of the voltage-programmable device. Furthermore, the large area of the link-plus-contacts results in high parasitic capacitance.
Thus, there is a need for better methods of manufacturing reliable, voltage programmable, metal-to-metal, link structures.